We have observed that, because of the normal operation of the pumps and other equipment used to place aqueous drilling fluids, workover fluids and fracturing fluids in wells, it is almost impossible to avoid including significant amounts of air in the fluid. While some of the oxygen may be dissolved, most of it remains in gaseous form. Oxygen in the air carried into the formation oxidizes the iron pyrites present on the surface of the shale in contact with the drilling or fracturing fluid. Chemical oxidizers in the form of gel breakers and biocides can also introduce oxygen into the fluid. When sulfur is released from the iron pyrite through oxidation of the iron pyrite, it tends to combine with other multivalent metals present in the formation, causing highly undesirable compounds to enter the produced fluids. Oxidation of iron pyrite not only results in troublesome sulfates of metals such as barium, calcium, and strontium, but also can generate iron oxides which will precipitate in the circulating fluid, causing deposits in piping, pumps and machinery, and even find its way into the produced hydrocarbons. Even very small amounts of iron in the oil or gas can be detrimental to catalysis and other processing steps for the hydrocarbon product. Moreover, after oxidation of iron pyrite, the mobility of both the iron and the sulfur from the previously situated iron pyrite is physically destructive to the shale.
Oxygen is almost always present in drilling, fracturing, and other aqueous well treatment fluids in amounts of at least 1%, generally in the range of 3% to 10% by volume; the actual volume of gaseous oxygen, and the portion dissolved, are functions of the pressure and temperature and to an extent the turbulence of the flow.
In the past, oxygen scavengers have been used to try to reduce the amount of dissolved oxygen that will react downhole. The most common oxygen scavengers, however, ammonium bisulfite and sodium bisulfite, both introduce free sulfate to the reservoir, where there is typically an excess of barium, strontium and calcium ionically bound to the substrate. Depending on their solubility constants and the common ion effect, the barium, strontium and calcium will more or less readily combine with the free sulfate to create water-insoluble scale.
It has recently become clear that virtually all shales encountered in drilling and fracturing operations, either in the recovery of oil or of natural gas, contain from 0.1% to 15% by volume iron pyrite distributed throughout. See, for example, John Kiefer and Warren H. Anderson “Foundation Problems and Pyrite Oxidation in the Chattanooga (Ohio) Shale, Estill County, Kentucky, Kentucky Geological Survey, University of Kentucky. Kiefer and Anderson have shown the detrimental effects of pyrite oxidation on the foundation of a building. When drilling through shale or an extended reach lateral (a horizontal wellbore in the shale that reaches out from the vertical wellbore) in the shale, similar shale stability problems are encountered. Wellbore stability in the shale is critical for successful drilling into any shale formation. If the shale loses its structural strength, the wellbore can collapse around the drill pipe, and in severe cases the hole being drilled must be abandoned along with the downhole equipment and the drill pipe. Because of ground stress, the action of water used in the drilling process, and wellbore pressures, shale stability problems are significantly greater in hydrocarbon recovery compared to the foundation problems encountered by Kiefer and Anderson. A typical reaction between pyrite and water is presented as 2FeS2+3H2O+3CaCO3+6O2→FeCa(SO4)+CO2+Fe. This paper deals with disintegration of shale around the foundation of a building, and thus does not involve a drilling or fracturing fluid, but the effects of oxygen studied is significant for drilling and fracturing operations. In part because of the abundance of pyrite in the shale, the described decomposition of the iron pyrite contributes significantly to the undesirable structural disintegration of the shale which plagues hydrocarbon recovery operations throughout the industry. Oxygen reacts both with iron and sulfur, causing rapid disintegration of the pyrite.
Since oxygen, both dissolved and undissolved (which together we call “free oxygen”), in the drilling or fracturing fluid is unavoidable, the damage caused by oxidation of iron pyrite alone can be very significant, resulting not only in the production of undesired multivalent metal compounds, but also, because the iron sulfide is a significant part of the shale, its molecular disruption significantly weakens the structure of the shale, exacerbating any independent hydration effects.
There is a need to cure the problem of formation damage following from the oxidation of iron pyrite in shale.
As will be seen below, a group of imidazoline derivatives has been found effective for constraining the degradation of shale and clay by inhibiting the oxidation of iron pyrite in the shale and clay. The reader may therefore be interested in reviewing the 1958 patent to Bohor et al, U.S. Pat. No. 3,389,750, describing the use of high molecular weight imidazolines to control formation damage in waterflooding practices. The Bohor patent lists some imidazoline derivatives, such as 1-hydroxyethyl-2-hexadecenyl imidazoline; 1-diethyldiamino-2-pentadecenyl imidazoline, 1-triethyltriamino-2-heneicosenyl imidazoline, 1-hydroxypropyl-2-octadecenyl imidazoline, and 1-hydroxypropyl-2-pentadecenyl imidazoline. Generally, these materials can be made by the reaction of an ethylene polyamine with a carboxylic acid. Imidazolines within a general imidazoline structure where the 1 position is occupied by a group of the formula —(CH2CH2NH)mC2H4NH2 and the 2 position is occupied by a hydrocarbon group derived from a straight chain fatty acid and containing from about 14 to about 22 carbon atoms, and a number of other imidazolines are attributed to DeGroote et al in U.S. Pat. No. 3,049,492, who recommend them for controlling sulfate-reducing bacteria in oil recovery methods. Even earlier, White et al, in U.S. Pat. No. 2,568,876, described the use of various imidazolines, for example the reaction products of oleic acid with tetraethylenepentamine, and, separately with diethylenetriamine, as corrosion inhibitors.